T cell tolerance not only is critical in preventing autoimmune diseases but also contributes to tumor tolerance. T cell anergy and regulatory T cells (Treg) are two major forms of peripheral T cell tolerance. However, the specific mechanisms that control T cell anergy and Treg are still not well understood. Rapamycin has been utilized as an immunosuppressant for decades. Its target, the mammalian target of rapamycin (mTOR), is a serine threonine kinase that plays crucial roles in multiple cellular processes by regulating nutrient uptake, transcription, translation, autophagy, and cell survival. mTOR forms two signaling complexes (mTORC1 and mTORC2) with distinct signaling properties and sensitivities to rapamycin. Recent evidence has established mTOR as a crucial regulator for effector T cell differentiation, T cell anergy, inducible regulatory T cell differentiation, and memory T cell responses to viral pathogens. However, the pathways leading to mTOR activation in T cells and the importance of tight control of mTOR activity remain poorly understood. The objective of this proposal is to improve the understanding of mTOR signaling and the importance of the tumor suppressor tuberous complex 1 (TSC1) as a critical regulator for mTOR in the control of T cell tolerance. Using genetically manipulated mice and cell line models, we have recently revealed that the RasGRP1-Ras- Mek1/2-Erk1/2 pathway is critical for both mTORC1 and mTORC2 activation in T cells following T cell receptor (TCR) stimulation. We and others have also demonstrated that TSC1 inhibits mTORC1 but promotes mTORC2 signaling in T cells and is important for normal T cell homeostasis. Our central hypotheses for this proposal are 1) multiple signaling cascades downstream of the TCR control mTOR activation and 2) TSC1 is a critical regulator for inducible Treg and T cell anergy by tight contro of mTORC1 and mTORC2 signaling. With strong preliminary data, we plan to test our hypotheses by pursuing three specific aims. In aim 1, we will investigate the mechanisms that control mTOR activation in T cells. We will test the hypothesis that Erk1/2 phosphorylate multiple substrates to promote mTORC1 and mTORC2 activation. In aim 2, we will determine how TSC1 controls T cell anergy. In aim 3, we will determine how TSC1 controls inducible Treg and helper T cell differentiation. The proposed studies will provide new insight into the understanding of mTOR signaling, the importance of TSC1 in T cells, and the mechanisms modulating T cell tolerance.